1. Field of the Invention
The present invention relates to a diffractive optical lens, and more particularly to a reflection and transmission diffractive optical lens having excellent focusing characteristics without aberrations when light is obliquely incident thereupon.
2. Description of the Prior Art
Since a diffractive optical lens exhibits excellent focusing characteristics with a thickness of a few .mu.m at most because of its grating construction, this type of lens has attracted attention as a miniaturized and light-weight lens.
Conventional reflection diffractive optical lenses are shown in FIGS. 1a and 1b. See T. Shiono, M. Kitagawa, K. Setsune and T. Mitsuyu: "Reflection micro-Fresnel lenses and their use in an integrated focus sensor", Appl. Opt., Vol. 28, No. 15, pp. 3434-3442 (1989).
This reflection diffractive optical lens has a transparent substrate 31 and a lens portion 38 provided on the substrate 31. The lens portion 38 has a grating zone 38a at its outer peripheral portion where a plurality of concentric circular grooves are provided and a core section 38b positioned at the center of its inner peripheral portion. The entire surface of the lens portion 38 is covered with a reflection layer 36. The grating zone 38a has a saw-tooth-shaped profile, and the distance between the grooves (grating period) becomes smaller toward the outer peripheral portion.
Conventional transmission diffractive optical lenses are shown in FIGS. 2a and 2b. See T. Fujita, H. Nishihara and J. Koyama: "Blazed gratings and Fresnel lenses fabricated by electron-beam lithography", Opt. Lett., Vol. 7, No. 12, pp. 578-580 (1982).
This transmission diffractive optical lens has a flat transparent substrate 41 and a lens portion 49 provided on the substrate 41. The lens portion 49 has a grating zone 49a where a plurality of concentric circular grooves are provided and a core section 49b in the same way as in the above-mentioned reflection diffractive optical lens. The grating zone 49a has a saw-tooth-shaped profile, and the distance between the grooves becomes smaller toward the outer peripheral portion.
In the above conventional reflection and transmission diffractive optical lenses, when light is vertically incident upon the surfaces of the substrates 31 and 41 on which the lens portions 38 and 49 are provided, the light is diffracted by the lens portions 38 and 49, whereby the light is satisfactorily focused. However, in the case of incident light 32 which is obliquely incident upon the surfaces of the substrates 31 and 41, aberrations are caused in reflected light 39 and transmitted light 47, thereby causing an out-of focus (obscure focal points 30 and 48). Thus, excellent focusing characteristics cannot be achieved.